lids separation processes 253 Table 9.7. Bulk density of some powders Bulk density Powder (kg m Powder 513 Milk 610 Wheat 785 Salt(granulated) 800 Cocoa Sugar(powdered) 480 Coffee (instant) 330 Wheat flou 480 Coffee(ground and roasted) 330 Yeast(bakers) 520 Corn starch Egg(whole) 340 From the data of Milson and Kirk(1980). From the data of Peleg (1983) Taken from Lewis(1990)(with courtesy of Prentice Hall) low bulk density of many food powders cannot be explained solely by geometrical considerations. As mentioned, most food powders are known to be cohesive, Therefore pen bed structures supported by interparticle forces are very likely to occur. Such materials are likely to have a low bulk density and high porosity. Factors that increase cohesiveness and interparticle forces are likely to decrease the bulk density. Moisture sorption tends to increased cohesiveness, mainly due to interparticle liquid bridges Anticaking agents are believed to work by reducing cohesive forces and thereby ncreasing bulk density. Peleg(1983)provides data for the cohesiveness of some powders together with the effects of some anticaking agents on the bulk density of some foo Powders can be compressed either by tapping or mechanical compression, as in tableting. The forces involved in compression are much higher than those in tapping or mechanical vibration The ratio of tapped bulk density to the loose bulk density is referred to as the Hausner ratio. Hayes(1987)quotes the following ranges, together with some values for some food powders free flowing 1.1-1.2 medium flowing 1.25-1.4 difficult >1.4 very difficult Hayes also refers to another index, termed the ' Novadel Tap Test, which is related to the percentage volume decrease on tapping. The larger the volume decrease, the poorer is the flowability. Peleg(1983)states that the Hausner ratio may be used for flowability index in powders, where friction is the major obstacle to flow, but that there is no evidence that it is useful for cohesive powders When powders are compressed the powder bed deforms and a number of mechanisms are involved, including spatial rearrangement of the particles without deformation
Solids separation processes 253 Table 9.7. Bulk density of some powders Bulk density Bulk density Powder (kg m-3) Powder (kg m-3> Oats 513 Milk 610 Wheat 785 Salt (granulated) 960 Flour 449 Sugar (granulated) 800 Cocoa 480 Sugar (powdered) 480 Coffee (instant) 330 Wheat flour 480 Coffee (ground and roasted) 330 Yeast (baker’s) 520 Corn starch 560 Egg (whole) 340 From the data of Milson and Kirk (1980). From the data of Peleg (1983). Taken from Lewis (1990) (with courtesy of Prentice Hall). low bulk density of many food powders cannot be explained solely by geometrical considerations. As mentioned, most food powders are known to be cohesive. Therefore open bed structures supported by interparticle forces are very likely to occur. Such materials are likely to have a low bulk density and high porosity. Factors that increase cohesiveness and interparticle forces are likely to decrease the bulk density. Moisture sorption tends to increased cohesiveness, mainly due to interparticle liquid bridges. Anticaking agents are believed to work by reducing cohesive forces and thereby increasing bulk density. Peleg (1983) provides data for the cohesiveness of some powders together with the effects of some anticaking agents on the bulk density of some food powders. Powders can be compressed either by tapping or mechanical compression, as in tableting. The forces involved in compression are much higher than those in tapping or mechanical vibration. The ratio of tapped bulk density to the loose bulk density is referred to as the Hausner ratio. Hayes (1987) quotes the following ranges, together with some values for some food powders: 1 .o- 1.1 free flowing 1.1-1.25 medium flowing 1.25-1.4 difficult >1.4 very difficult. Hayes also refers to another index, termed the ‘Novadel Tap Test’, which is related to the percentage volume decrease on tapping. The larger the volume decrease, the poorer is the flowability. Peleg (1983) states that the Hausner ratio may be used for flowability index in powders, where friction is the major obstacle to flow, but that there is no evidence that it is useful for cohesive powders. When powders are compressed the powder bed deforms and a number of mechanisms are involved, including spatial rearrangement of the particles without deformation
254 M.J. Lewis together with those brought about by fragmentation and plastic deformation of the particles. For cohesive powders, the open structure supported by interparticle forces is relatively easily overcome by the compressive force and there is a relatively large change f bulk density with pressure. Non-cohesive powders show relatively little change of bu density with pressure An empirical relationship of the following form is found to fit experimental data well =a+b log s (96 where PB is the bulk density, a and b are constants and s is the shear stress. The constant b is defined as the compressibility. High values of b indicate a cohesive powder, whereas low values indicate a non-cohesive powder. Some values for different powders are given by Peleg(1983). The use of anticaking agents was found to reduce the compressibility 9. 2.7 Flowability The flowability of powders is very important in their handling. Some indices of flow- ability have already been discussed Generally flowability increases with increasing particle size and decreasing moisture content.As well as compressibility and cohesiveness, other factors used to assess flow- ability are as follows: Slide angle. This is measured by placing the powder sample on a flat smooth horizon tal surface, which is then slowly inclined until the powder begins to move. The angle at which movement occurs is known as the slide angle. Angle of repose. This is useful in the design of powder handling systems. Its value depends upon the method of determination, which is usually by forming a heap. Other methods involve a bed rupture or a rotating drum method. Its magnitude is affected by frictional forces and interparticle attractive forces, which become dominant in cohe sive powders. ccording to CarT(1976), angles of up to 35 indicate free flowability: 35-450 dicates some cohesiveness; 45-55%indicates cohesiveness or loss of free flowability >55 indicates very high cohesiveness, very limited or zero flow These parameters are empirical in nature and often the results are not applicable, when onditions are changed(Peleg, 1977) Peleg(1977)and Schubert (1987a) have described a more fundamental method for looking at the flow behaviour of powders, based on the work of Jenike, described by Leniger and Beverloo(1975). A flow cell is used, where the powder is first consolidated to a particular bulk density and porosity(see Fig. 9.3(a)). It is then subjected to a oppressive force(M)and the shear force(S) required to cause the powder to yield and shear is determined. These readings are converted to a normal stress(o)(N/A)and a shear stress(r)(S/A). This procedure of determining the shear stress is repeated for a number of different normal stress values. The information is presented on a plot of shear stress against normal stress and gives the yield locus, for that particular porosity Figure 9.3(b)shows the data obtained for a non-cohesive powder, which can be characterised by the angle of friction (a). Also in all cases a large angle of friction
254 M. J. Lewis together with those brought about by fragmentation and plastic deformation of the particles. For cohesive powders, the open structure supported by interparticle forces is relatively easily overcome by the compressive force and there is a relatively large change of bulk density with pressure. Non-cohesive powders show relatively little change of buik density with pressure. An empirical relationship of the following form is found to fit experimental data well: p~ =a + b log s (9.6) where p~ is the bulk density, a and b are constants and s is the shear stress. The constant b is defined as the compressibility. High values of b indicate a cohesive powder, whereas low values indicate a non-cohesive powder. Some values for different powders are given by Peleg (1983). The use of anticaking agents was found to reduce the compressibility. 9.2.7 Flowability The flowability of powders is very important in their handling. Some indices of flowability have already been discussed. Generally flowability increases with increasing particle size and decreasing moisture content, As well as compressibility and cohesiveness, other factors used to assess flowability are as follows: Slide angle. This is measured by placing the powder sample on a flat smooth horizontal surface, which is then slowly inclined until the powder begins to move. The angle at which movement occurs is known as the slide angle. Angle of repose. This is useful in the design of powder handling systems. Its value depends upon the method of determination, which is usually by forming a heap. Other methods involve a bed rupture or a rotating drum method. Its magnitude is affected by frictional forces and interparticle attractive forces, which become dominant in cohesive powders. According to Carr (1976), angles of up to 35' indicate free flowability; 35-45" indicates some cohesiveness; 45-55' indicates cohesiveness or loss of free flowability ; >55' indicates very high cohesiveness, very limited or zero flow These parameters are empirical in nature and often the results are not applicable, when conditions are changed (Peleg, 1977). Peleg (1977) and Schubert (1987a) have described a more fundamental method for looking at the flow behaviour of powders, based on the work of Jenike, described by Leniger and Beverloo (1975). A flow cell is used, where the powder is first consolidated to a particular bulk density and porosity (see Fig. 9.3(a)). It is then subjected to a compressive force (N) and the shear force (S) required to cause the powder to yield and shear is determined. These readings are converted to a normal stress (0) (N/A) and a shear stress (7) (SIA). This procedure of determining the shear stress is repeated for a number of different normal stress values. The information is presented on a plot of shear stress against normal stress and gives the yield locus, for that particular porosity. Figure 9.3(b) shows the data obtained for a non-cohesive powder, which can be characterised by the angle of friction (a). Also in all cases a large angle of friction
Solids separation processes 255 Fig. 9.3. Solid characterisation:(a) Jenike flow cell; (b)normal stress against shear stress, for a on-cohesive powder, a= angle of friction;(c) compacted to different initial porosities; po unconfined yield stress (e)and major consolidation stress(on) indicating high interparticle friction, does not always mean poor flowability, for example dry sand has a high value but fl te well Figure 9.3(c) shows the locus for a cohesive powder, at a particular porosity However, if the porosity of the sample is increased, the yield locus will change. There fore there are a family of curves at different porosities, also the curves do not pass through the origin. This yield locus data therefore describes the flow behaviour of powders. This data is used to determine the unconfined yield stress (c) and the major consolida- tion stress (o,), by application of Mohrs circles(see Peleg, 1977; Schubert, 1987a Leniger and beverloo, 1975) The ratio of o, is termed the Jenike flow function, which has also been used as an indicator of the flowability of powders. Its values correspond to the following character very cohesive, non-flowing 2-4 cohesive 10 free flowing. This more fundamental information is extremely useful for designing hoppers, bins, pneumatic conveying systems and dispensers. Similar measurements can be made using a more sophisticated annular flow cell, which is capable of reliable shear force determina- tions at lot The hydrodynamics of powder flow are different to that for liquids. The pressure does not increase linearly with height, rather it is almost independent. Also they can resist appreciable shear stress and can, when compacted, form mechanically stable structures
Solids separation processes 255 tN --t Tp 0 0 Tp (4 (b) (c) (4 Tb Fc (J1 (J Fig. 9.3. Solid characterisation: (a) Jenike flow cell; (b) normal stress against shear stress, for a non-cohesive powder, a = angle of friction; (c) yield locus for a cohesive powder for powders compacted to different initial porosities; porosity 1 > 3; (d) Mohrs circles, showing the unconfined yield stress (f,) and major consolidation stress (01). indicating high interparticle friction, does not always mean poor flowability, for example dry sand has a high value but flows quite well. Figure 9.3(c) shows the yield locus for a cohesive powder, at a particular porosity. However, if the porosity of the sample is increased, the yield locus will change. Therefore there are a family of curves at different porosities. Also the curves do not pass through the origin. This yield locus data therefore describes the flow behaviour of pow ders. This data is used to determine the unconfined yield stress (f,) and the major consolidation stress ((T~), by application of Mohrs circles (see Peleg, 1977; Schubert, 1987a; Leniger and Beverloo, 1975). The ratio of ol/fc is termed the Jenike flow function, which has also been used as an indicator of the flowability of powders. Its values correspond to the following characteristics: <2 very cohesive, non-flowing 24 cohesive 4-10 easy flowing >10 free flowing. This more fundamental information is extremely useful for designing hoppers, bins, pneumatic conveying systems and dispensers. Similar measurements can be made using a more sophisticated annular flow cell, which is capable of reliable shear force determinations at low normal stresses. The hydrodynamics of powder flow are different to that for liquids. The pressure does not increase linearly with height, rather it is almost independent. Also they can resist appreciable shear stress and can, when compacted, form mechanically stable structures
256 M.J. Lewis that may halt flow. Also any pressure or compaction can increase the mechanical strengt and hence the flowability 9.3 SEPARATION OF PARTICULATES AND POWDERS This section will be most concerned with the separation or recovery of solids from within a solid matrix or from a particulate system. The main emphasis will be those in fine particulate form, so the production of material in a form suitable for separations is often crucial for the process. In this respect size reduction and milling equipment is important. 9.3.1 Size reduction Size reduction is a very important preliminary operation for separation processes for many cereals, legumes and other commodity crops, as well as for extraction operations g fruit juice expulsion or oil ex Sugar is one example of a commodity that comes in a range of particle sizes, e.g caster, and icing measurements of diffe ugars is cited by Hayes (1987) The term ' crushing olied to the reduction of coarse material down to a size of about 3 mm, whereas 'grinding is commonly used for the production of finer powdered material The degree of size reduction can be characterised by the size reduction ratio(SRR) SRRE rage size of feed (9.7) Several stages may be required if the overall size reduction is large The main forces involved are compressive forces, impact forces and shear or attrition forces. Usually there is a predominant force involved for each type of equipment,a though the other forces may be involved to a lesser extent. The fracture resistance in- creases with decreasing particle size Aspects which need to be considered in the selection of the most appropriate equip- ment for size reduction are the particle size range required and the hardness of the material Hardness can be measured in Mohs, whose scale ranges between 0 and 8.5. On this hardness scale, most foods are either very soft(<1.5 Moh); soft(1. 5 to 2.5 Moh)or medium hard(2.5 to 4.5 Moh). More details are provided by Hayes(1987)and christison (1991) Very soft materials such as dried fruit, dried plant material, meat and fish may be processed with a Colworth stomacher down to 100 um, or high-speed cutters, such as a bowl choppers Other mills for processing grain cereals, legumes, salt, and sugar include the follow 1)Hammer mills. These are very much general-purpose mills. Size reduction is mainly due to impact forces. They are widely used for peppers and other spices, sugar and dried milk powder. (2) Roller mills. These can be one or several sets of rollers; size reduction is by
256 M. J. Lewis that may halt flow. Also any pressure or compaction can increase the mechanical strength and hence the flowability. 9.3 SEPARATION OF PARTICULATES AND POWDERS This section will be most concerned with the separation or recovery of solids from within a solid matrix or from a particulate system. The main emphasis will be those in fine particulate form, so the production of material in a form suitable for separations is often crucial for the process. In this respect size reduction and milling equipment is important. 9.3.1 Size reduction Size reduction is a very important preliminary operation for separation processes for many cereals, legumes and other commodity crops, as well as for extraction operations, e.g. tea and coffee, or expression processes, e.g. fruit juice expulsion or oil extraction. Sugar is one example of a commodity that comes in a range of particle sizes, e.g. granular, caster, and icing sugars. Some data on sieve size measurements of different sugars is cited by Hayes (1987). The term ‘crushing’ is applied to the reduction of coarse material down to a size of about 3 mm, whereas ‘grinding’ is commonly used for the production of finer powdered material. The degree of size reduction can be characterised by the size reduction ratio (SRR), where (9.7) average size of feed average size of product SRR = Several stages may be required if the overall size reduction is large. The main forces involved are compressive forces, impact forces and shear or attrition forces. Usually there is a predominant force involved for each type of equipment, although the other forces may be involved to a lesser extent. The fracture resistance increases with decreasing particle size. Aspects which need to be considered in the selection of the most appropriate equipment for size reduction are the particle size range required and the hardness of the material. Hardness can be measured in Mohs, whose scale ranges between 0 and 8.5. On this hardness scale, most foods are either very soft (~1.5 Moh); soft (1.5 to 2.5 Moh) or medium hard (2.5 to 4.5 Moh). More details are provided by Hayes (1987) and Christison (1991). Very soft materials such as dried fruit, dried plant material, meat and fish may be processed with a Colworth stomacher down to 100 pm, or high-speed cutters, such as a bowl choppers. Other mills for processing grain cereals, legumes, salt, and sugar include the following: (1) Hummer mills. These are very much general-purpose mills. Size reduction is mainly due to impact forces. They are widely used for peppers and other spices, sugar and dried milk powder. (2) Roller mills. These can be one or several sets of rollers; size reduction is by
compressive forces; size reduction ratio is usually below 5. These are widely used for the milling of wheat and refining of chocolate. ( Size range 10-1000 um. (3) Disc attrition mills. These come in a number of designs. Simple disc mills have two discs, one of which is stationat 1. The feed material enters at the centre of the discs and the other mot is relatively slot with a peripheral velocity of 4-8 ms and the discs are profiled in such a way as to cause grinding to occur as the material falls adially across the grinding discs (Size range down to 100 um.) On the other hand, impact pulveriser mills, such as pin disc or stud mills, operate at high rotational speeds, creating peripheral velocities up to 200 m s. In this case the discs contain pins or studs, which intermesh In the simple design there is one stationary and one moving disc, whereas in other designs both discs move. These types of mill can produce very fine powders, suitable for air classification( see Section 9. 4) Another high-speed mill is the high-speed rotor mill, which is a variant of the hammer mill. A rotor with a series of hardened blades rotates at speeds in excess of 15 000 r.P.m and the fines pass through a sieve ring, fitted round the circumference (4)Ball mills. This is a tumbling mill and is used for very fine grinding processes. It omprises a horizontal slow-speed rotating cylinder which contains steel balls of flint stones; the balls are normally 25-150 mm in diameter. The mechanism is by impact and shear. The optimum speed of rotation is about 75% of the critical speed, which is defined as the speed which causes the steel balls to centrifuge Two or more mill types may be required to achieve the desired level of size reduction The size reduction achieved often depends on whether the discharge product is released immediately or whether it is restricted by use of a screen. In the latter case the residence time within the action zone is increased until the particle is smaller than that of the screen. A third alternative is to allow all the particles to leave unrestricted and to separate them externally, recycling oversize particles for further milling The particle size required also affects the cost of milling and the energy requirement the latter is based on the following equation de K dD D where dE is the energy required to produce a small change in diameter dd and km is characteristic of the material. The three main equations result from different values of n (Note: n is a power-law exponent. n=l:E=Km In(D/D2] Kick's law n=号:E=2K n=2: E=Kmll-1 Fitting
Solids separation processes 257 compressive forces; size reduction ratio is usually below 5. These are widely used for the milling of wheat and refining of chocolate. (Size range 10-1000 pm.) (3) Disc attrition mills. These come in a number of designs. Simple disc mills have two discs, one of which is stationary and the other moving. The speed is relatively slow, with a peripheral velocity of 4-8 m s-'. The feed material enters at the centre of the discs and the discs are profiled in such a way as to cause grinding to occur as the material falls radially across the grinding discs. (Size range down to 100 pm.) On the other hand, impact pulveriser mills, such as pin disc or stud mills, operate at high rotational speeds, creating peripheral velocities up to 200 m s-*. In this case the discs contain pins or studs, which intermesh. In the simple design there is one stationary and one moving disc, whereas in other designs both discs move. These types of mill can produce very fine powders, suitable for air classification (see Section 9.4). Another high-speed mill is the high-speed rotor mill, which is a variant of the hammer mill. A rotor with a series of hardened blades rotates at speeds in excess of 15 000 r.p.m. and the fines pass through a sieve ring, fitted round the circumference. (4) Bull mills. This is a tumbling mill and is used for very fine grinding processes. It comprises a horizontal slow-speed rotating cylinder which contains steel balls of flint stones; the balls are normally 25-150 mm in diameter. The mechanism is by impact and shear. The optimum speed of rotation is about 75% of the critical speed, which is defined as the speed which causes the steel balls to centrifuge. Two or more mill types may be required to achieve the desired level of size reduction. The size rediction achieved often depends on whether the discharge product is released immediately or whether it is restricted by use of a screen. In the latter case the residence time within the action zone is increased until the particle is smaller than that of the screen. A third alternative is to allow all the particles to leave unrestricted and to separate them externally, recycling oversize particles for further milling. The particle size required also affects the cost of milling and the energy requirement: the latter is based on the following equation: dE - K, dD D" -- - where dE is the energy required to produce a small change in diameter dD and K, is a characteristic of the material. The three main equations result from different values of n. (Note: n is a power-law exponent.) n = 1: E = K,, ln[Dl/D2]; Kick's law Bond's law n = 7: 3 E = 2K,[L--&] D20.5 n=2: E = Kn,[&-+] Rittinger's law